A friend of mine recently found out her infant daughter has several heart defects and will need many surgeries throughout her life. Plans are being made for a silent auction to benefit her family. I would really like to help but the only thing I can think of is to somehow donate a cake. What do you think is the best way to do this? I am not a professional so I think I would be more comfortable making a cake and actually having it there at the auction. That way people can actually see what they are bidding for. But I wonder if it would be more profitable for the auction to just have pictures of my cakes available and offer a specific size cake, thus allowing the bidder to pick the design and when they want it? Also, I am allowed to legally donate a cake right?

I donated a cake to a fundraising golf outing a few weeks ago for a raffle. I made a dessert cake so that it would be appropriate for anybody that received it.

I think you are likely to get higher bids if you show bidders photos of your work, or maybe you can make a dummy cake, than if they were bidding on an actual cake on a table. I would prefer to bid on a cake decorated for me at a date of my choice. Hopefully whoever wins the cake will not want you to do something terribly difficult or above your skill level.
I agree you should state serving amounts (or a similar size to the dummy you have presented if you choose to) and a date by which the cake must be ordered/delivered, 6 months or whatever you are comfortable with.
I can't say if you can legally donate a cake, the US has some complicated laws but I'm sure someone else can help.

For a silent auction, you can state what kind of cake they are bidding on. Be as specific as you want.

For example a 9 x 13" 1-layer cake with buttercream decorations only, or a 9" fondant cake with gumpaste flowers. That way you do not have to worry about someone asking for a cake above your skill level.

Definitely have pictures of your work available so they have an idea of what they might be getting.

I did this exact thing recently. I made a large posterboard with photos of some of the cakes I've done before and put a letter in the middle with my name and something saying I would make "any" cake anyone wanted. I felt pretty safe with saying "any" because I live in a very rural area and most people here have no idea what type of cakes can be made.
I also attached an envelope for the winner with more information to contact me and note saying how I really appreciated the donation for the cause. (mine was an 18 month girl with brain cancer)
It turned out that the winner wanted a single "wedding" cake and cupcakes.
I love doing donated cakes. You don't have to worry about the price, getting paid and it just makes you feel good.

I think if you aren't legal then you should offer a cake there.
You could also offer cookie bouquet.

I have created a cake silent before and will be doing so again in about another month. I made a display of various cake styles. Childs birthday, sweet 16 and wedding cake. ( I did this because not everyone is getting married and it showed range of use ).Along with those was an outline of what they were bidding on....how many tiers, max value, etc.. I also included an expiration date as well as the ability to transfer the prize if they wanted to gift it to another person. It went EXTREMELY well! I received many orders from attendees that saw my display but did not win. I'm really looking forward to the next one!

I recently donated a cake for a fund raiser at my son's school.....it was also a silent aution.
I made a 3 tier dummy cake, just to represent my quality of work/skill... I intentionally made the cake especially large so as to attract attention to my cake in the midst of hundreds of items up for auction.....
I had a nicely written explanation sitting near that cake that stated:
THIS CAKE IS NOT REAL... the person who wins the aution will choose the theme/decorations for a cake, white or chocolate, and the cake will serve @ 30 people, it was autioned off but I never heard back from the 'winner'!!
I STILL have the dummy cake sitting in my basement and... someone PINCHED the cake... presumably to see if it was real!! I didn't think I needed to make a note that read "DO NOT TOUCH"... but I was wrong!!